(i) Technical Field
The present invention relates to a driving method of writing an image on a liquid crystal device, and an apparatus for driving the liquid crystal device.
(ii) Background Art
For a reason, such as earth environment preservation such as forest resources protection, or office environment improvements called “space save”, an expectation for a rewritable marking technique as a hard copy technology shifting to paper is high.
Hard copy of paper has excellent convenience, which does not belong to a conventional electronic display, such as (1) it provides bright and high-contrast reflective full-color display, convenient reading, and a high information display density, (2) it has a light, thin and flexible structure, can be seen at a comfortable posture, and provides a wide selection range of irradiation conditions, (3) it has a memory characteristic in display, can provide information display and retention with no power, and enables less eye fatigue through flickless display, and (4) it can provide easy look through simultaneous display of plural sheets at low cost, and convenient comparison and browsing of information, and so on.
For this reason, the paperlessness of office has not been accelerated, as expected, resulting in a behavior in which information displayed on an electronic display is printed on paper as a hard copy and is read. Therefore, a display medium replacing paper requires the reappearance of various convenience unique to the above paper document in addition to a rewriting function for realizing resource saving and waste saving.
Research has been done on a variety of rewritable marking techniques with further improved convenience. As one direction, a technique by a liquid crystal device in which liquid crystal and an optical conductor are stacked enables repetitive writing/erase to be implemented, and has been in the spotlight as realizing other excellent characteristics.
The introduction of this technique will be described below. A planar phase represented by cholesteric liquid crystal (chiral nematic liquid crystal) induces a selective reflection phenomenon in which in a state where incident light parallel to screw axes is divided into right linear light and left linear light, the screws interfere and reflect a circular polarization component parallel to a twist direction and permit transmission of the remaining light. A center wavelength λ and a reflection wavelength width a of reflected light has a relationship of λ=n·p, and Δλ=Δn·p, respectively, wherein p is a screw pitch, n is an average refractive index within a plane orthogonal to the screw axes, and Δn is birefringence. The reflected light by the cholesteric liquid crystal layer of the planar phase is tinged with a clear color dependent on the screw pitch.
Cholesteric liquid crystal having positive dielectric anisotropy exhibits three states: namely, a planar phase in which screw axes become perpendicular to the surface of a cell to thus induce the above-described phenomenon of selectively reflecting incident light as shown FIG. 16A, a focal conic phase in which screw axes become essentially parallel to the surface of a cell to thus permit transmission of incident light while the incident light is slightly scattered forward as shown in FIG. 16B, and a homeotropic phase in which the screw structures become untied and liquid-crystal directors are oriented in the direction of an electric field to thus allow transmission of incident light.
Of the above three phases, the planar phase and the focal conic phase can be bistably present in a field-free state. Consequently, the phase of cholesteric liquid crystal is not uniquely determined with respect to the strength of an electric field applied to a liquid crystal layer. When the planar phase is an initial state, the phase changes from the planar phase, the focal conic phase, and the homeotropic phase, in this sequence with an increase in the strength of the electric field. When the focal conic phase is an initial state, the phase changes from the focal conic phase to the homeotropic phase in this sequence in accordance with an increase in the strength of the electric field.
When the strength of the electric field applied to the liquid crystal layer is abruptly reduced to zero, the planar phase and the focal conic phase remain intact, and the homeotropic phase changes to the planar phase.
Consequently, immediately after application of a pulse signal the cholesteric liquid crystal layer exhibits a switching behavior such as that shown in FIG. 17. In the case where an initial state is the planar phase, when the voltage of the applied pulse signal is Vph or more, there is achieved a selective reflection state obtained as a result of the homeotropic phase having changed to the planar phase, as indicated by the white circles. When the voltage ranges between Vpf and Vph, a transmission state realized by the focal conic phase is achieved. When the voltage is Vpf or less, a state realized before application of the pulse signal becomes continual; namely, the selective reflection state realized by the planar phase is achieved.
Meanwhile, in the case where an initial state is the focal conic phase, when the voltage of the applied pulse signal is Vfh or more, there is achieved the selective reflection state obtained as a result of the homeotropic phase having changed to the planar phase, as indicated by the black circles. When the voltage of the applied pulse signal is Vfh or less, a state realized before application of the pulse signal becomes continual; namely, the transmission state realized by the focal conic phase is achieved.
In the drawing, the vertical axis represents normalized reflectance. Reflectance is normalized on the assumption that the maximum reflectance is 100 and that the minimum reflectance is zero. It is assumed that a phase change of the planar phase and the homeotropic phase assumes a threshold voltage of Vph, a phase change of the planar phase and the focal conic phase assumes a threshold voltage of Vpf, and a phase change of the focal conic phase and the homeotropic phase assumes a threshold voltage of Vfh. Furthermore, there is a transition region between the planar phase, the focal conic phase and the homeotropic phase. Normalized reflectance defined as a threshold level is represented by dotted lines. In the drawing, normalized reflectance 50 is indicated by a threshold level.
The liquid crystal device of this technique implements a monochrome display of black and white with a memory characteristic in a field-free state or a color display with a memory characteristic in a field-free state, by means of switching (A) a selective reflection state realized by the planar phase and (B) a transmission state realized by the focal conic phase by utilization of the bistable phenomenon of the cholesteric liquid crystal.
Furthermore, in the liquid crystal device of this technique, the self-retention type liquid crystal compound and the organic photoconductor can be formed by coating an undiluted solution or through a laminate process. The liquid crystal device can be fabricated conveniently at low cost. Furthermore, both the self-retention type liquid crystal compound and the organic photoconductor can easily realize resolutions required for hard copy and, therefore, can enhance display resolving power of the liquid crystal device.
The liquid crystal device of this technique can perform writing of an image by forming an image although exposure is not performed over the whole surface at the same time and scanning the surface of the liquid crystal device by use of an exposure device of a scanning system (for example, a laser light exposure device) or a light-emitting diode array.
There is shown in FIG. 18 a diagram showing a shape in which an image is written using the exposure device of the scanning system by means of a general method of driving a liquid crystal device. As illustrated in FIG. 18, the liquid crystal device of this technique includes a display layer (i.e., a liquid crystal layer) and an OPC layer (i.e., a photoconductive layer), which are laminated with, for example, a light-blocking layer intervened therebetween between a pair of electrode substrates. After the whole surface of the display layer is reset to the planar phase, the surface on the OPC layer side is exposed like an image using an exposure device, such as a line head or a beam scanner, with a bias voltage being applied to the pair of the electrodes, thereby recording a writing image on the surface.
As mentioned above, the liquid crystal layer at the time of writing forms a desired writing image by obtaining contrast between a portion at which a phase changes from the planar phase to the focal conic phase and a portion at which a phase does not change from the planar phase to the focal conic phase depending on whether exposure is performed. A phase change from the planar phase to the focal conic phase needs some degree of time. Specifically, the liquid crystal layer completes the phase change by taking several hundreds of ms (approximately 200 ms or more). Accordingly, since a writing time of several hundreds of ms is taken every scan line (or 1 pixel), a great period of time is taken to write the whole surface of the liquid crystal device. For this reason, it could be said that the above technique is not sufficient regarding practical use in writing.
There is also a technique for significantly shortening a writing time taken every scan line (or 1 pixel) by reducing time necessary for a phase change.